The present invention relates to devices for actuating an electrical switch in response to a predetermined temperature, such devices being commonly referred to as thermostats. One well known type of thermostat employs a liquid filled sensing bulb connected via a capillary tube to a pressure responsive diaphragm which exerts a force on a moveable member for switch tripping in response to expansion of the liquid sealed within the capillary tube and bulb.
In such bulb-and-capillary tube type thermostats, the moveable output member is commonly capable of providing substantial force, but very little motion from the fluid expansion generated in response to the bulb sensing a desired temperature level. Accordingly, it has been past practice to employ a lever means to amplify the motion of the thermally responsive output member for providing sufficient motion to effect actuation or deactuation of the electrical switch. In arrangements where such a lever has been employed, it is known to have the thermally responsive output member contact a portion of the lever arm and the lever biased about a fulcrum to maintain lever contact with the thermally responsive output member. In such an arrangement, it is necessary to calibrate the switch position with respect to movement of the lever arm in order that the arm will effect switch operation at the desired temperature. Once such a calibration has been made, however, the calibration is suitable only for an initial temperature corresponding to the ambient temperature at which the switch was calibrated. If the ambient temperature surrounding the switch is substantially changed from that which calibration was performed, the calibration is shifted by virtue of thermal expansion or contraction of the fluid in the fluid filled reservoir and capillary causing a change in the value of the sensed temperature at which switch actuation occurs.
Attempts have been made heretofore to provide ambient temperature compensation for the aforementioned types of lever actuated thermostats in order to prevent the shift in calibration caused by changes in ambient temperature. It is known to provide such a thermostat having a switch actuating lever formed of a bimetal material such that warpage or deformation of the bimetal with changes in ambient temperature is designed to counteract the shift in the calibration and thereby maintain the trip point of the thermostat switch constant over a range of expected service ambient temperatures.
However, calibration of a thermostat having a bimetal switch actuating arm requires that after switch assembly the arm be subsequently bent or otherwise altered in configuration to accomplish the initial temperature calibration. This has been found to be a complicated and costly operation to perform during manufacturing. Thus, it has long been desired to find a way or means of providing ambient temperature compensation in a lever acutated thermostatic switching device which would render the assembled thermostat easy to calibrate during manufacturing.